On Mar. 26, 2013, the Advanced Television Systems Committee (ATSC), which proposes terrestrial broadcasting digital television standards in the U.S., announced a call for proposals for the next generation (named ATSC 3.0) physical layer. ATSC 3.0 will provide even more services to the viewer and increased bandwidth efficiency and compression performance. This will require breaking backward compatibility with the currently deployed version, ATSC A/53, which comprises an 8-VSB (8 level, Vestigial Sideband) modulation system. ATSC 3.0 is expected to emerge within the next decade and it intends to support delivery to fixed devices of content with video resolutions up to Ultra High Definition 3840×2160 at 60 frames per second (fps). The intention of the system is to support delivery to portable, handheld and vehicular devices of content with video resolution up to High Definition 1920×1080 at 60 fps. The system is also expected to support lower video resolutions and frame rates.
One of the main issues associated with the current ATSC standard is the vulnerability of the 8-VSB modulation system to multipath propagation and Doppler Effect. These impairments are present in the broadcast transmission environment, particularly in large metropolitan cities, and in the delivery to portable/handheld/vehicular devices (which ATSC intends to support). It is a consensus that multi-carrier modulation systems like, for example, the OFDM (orthogonal frequency division multiplex) modulation, are better choices of modulation to combat these impairments.
OFDM is a method of encoding digital data on multiple carrier frequencies. In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not required. The orthogonality allows for efficient modulator and demodulator implementation using the FFT (Fast Fourier Transform) algorithm on the receiver side, and inverse FFT on the transmitter side. In particular, the size of the FFT identifies the number of carriers in the OFDM modulation system. Frequency selective channels are characterized either by their delay spread or coherence bandwidth. In a single carrier system like 8-VSB, a single fade or interference can cause the whole link to fail, but in multi-carrier systems, like OFDM, only a few of the total sub carriers will be affected. This way, multipath fading can be easily eliminated in OFDM, with simpler equalization techniques than in single carrier systems.
The OFDM modulation is adopted in other digital terrestrial television standards, e.g., the DVB-T/DVB-T2 standards in Europe, and the ISDB-T standard in Japan. DVB-T, the 1st generation of European DTT (Digital Terrestrial Television), is the most widely adopted and deployed standard. Since its publication in 1997, over 70 countries have deployed DVB-T services and 45 more have adopted (but not yet deployed) DVB-T. This well-established standard benefits from massive economies of scale and very low receiver prices. Like its predecessor, DVB-T2 uses OFDM (orthogonal frequency division multiplex) modulation with a large number of sub-carriers delivering a robust signal, and offers a range of different modes, making it a very flexible standard. DVB-T2 uses the same error correction coding as used in DVB-S2 and DVB-C2: LDPC (Low Density Parity Check) coding combined with BCH (Bose-Chaudhuri-Hocquengham) coding, offering a very robust signal. The number of carriers, guard interval sizes and pilot signals can be adjusted, so that the overheads can be optimized for any target transmission channel. DVB-T2 offers more robustness, flexibility and at least 50% more efficiency than any other DTT system. It supports SD, HD, UHD, mobile TV, or any combination thereof.
Within the DVB family there is a standard specifically for metadata, or Service Information (SI), also called DVB-Sl. The standard allows for SI to accompany broadcast signals and is intended to assist the receiver/decoder and viewers to navigate through the growing array of digital services on offer. Within DVB-SI, the Event Information Table (EIT) is specifically important as a means of communicating program (“event”) information. The EIT can be used to give information such as the program title, start time, duration, a description and parental rating. It is also possible to classify programs using what are known as “content descriptors”, identifying the content from a set of categories and sub-categories.
The current DVB-T2 system contains a feature called Physical Layer Pipes (PLP), which represent different services or virtual channels within the data stream of one physical channel (or spectral band). A DVB-T2 signal may contain multiple PLPs. This feature allows for differing types of data to be sent with differing data rates and amounts of error correction. Further up the communication stack, there is information in the DVB-SI information that maps the PLPs to their content. For example, the DVB-SI may describe which PLP contains a program video and which other PLP contains a program audio.
When a new broadcast system is deployed, as it will eventually be the case for ATSC 3.0, one issue to consider is whether distinct broadcasters are sharing physical channels. For many years, television broadcasting has been characterized by having one broadcaster transmitting one TV program over one physical channel. With the introduction and transition to ATSC broadcasting, this mindset has started to change. The availability of multiple programs on a single channel as well as the introduction of integrated program guides has enhanced the importance of the broadcaster while loosening the link between the broadcaster and the physical channel. This trend should continue and be enhanced with the introduction of ATSC 3.0. The end consumer does not care over which physical channel a service is delivered.
Another issue to consider is the requirement for broadcast systems to carry information related to the Emergency Alert System (EAS). The EAS is an American national public warning system that requires broadcasters, cable television systems, wireless cable systems, satellite digital audio radio service (SDARS) providers, and direct broadcast satellite (DBS) providers to provide the communications capability to the President to address the American public during a national emergency. The system may also be used by state and local authorities to deliver important emergency information, such as AMBER alerts (child abduction emergency) and weather information targeted to specific areas (e.g., tornadoes, blizzards, floods, etc.). Other countries may adopt similar systems.
For the DVB-T2 standard, a problem may occur if more than one broadcaster is sharing a physical channel. Each broadcaster may have its own set of DVB-SI information. Currently, there is no method defined in the DVB-T2 standard to describe which PLPs/services belong to which broadcaster and where to find each broadcaster's DVB-SI or equivalent information (e.g., PSIP). A single PSIP table could still be used to describe all of the programs from different stations/broadcasters, however, this would require them to cooperate at a higher level (e.g., one of them would have to send their programming info ahead of time to the other to make a single PSIP table), which is not ideal.
The present principles attempts to encourage and facilitate cooperation among broadcasters by proposing ways to convey important system information such as the mapping between a broadcaster and its PLPs/services carried in a multi-carrier modulation system for which the data stream is organized in multiple physical layer pipes (PLP). In addition, special messages like the EAS and Amber Alert messages may also be conveyed by the same mechanism.